Wireless communication systems employ simulcast to provide service over large areas or in hard to cover places such as deep inside buildings. With simulcast, transceivers at multiple base sites simultaneously transmit the same signal. As communication units, such as portable or mobile radios, move over the area covered by the wireless communication system, they are able to receive the signal from one or more of the base site transceivers. Because of the overlap of the areas covered by each of the base sites, there is a high probability that the communication units will be able to successfully receive the signal.
FIG. 1 shows an example of a simulcast system 100. The simulcast system 100 includes a prime site 105 and three remote sites 107, 109, 111 electrically coupled to a network 113. The prime site 105 and remote sites 107, 109, 111 are collectively referred to as a simulcast site. The prime site contains a simulcast site controller 120 and three comparators 122, 124, 126. Each of the remote sites 107, 109, 111 contains three simulcast stations 130–132, 133–135 and 136–138 respectively. The simulcast stations 130–138 are transceivers that transmit and receive signals to and from communication units (not shown) within a coverage area. The coverage areas of the remote sites 107, 109, 111 are illustrated by the circles on FIG. 1. The circles overlap to show that the coverage areas of the simulcast stations 130–138 at the different remote sites 107, 109, 111 overlap. When communication units (not shown) are in overlapping areas 140, 142, they may receive and transmit signals to and from more than one of the remote sites 107, 109, 111. For example, when a communication unit is in the overlapping area 140, it is able to receive and transmit signals to the remote site 107 and the remote site 109.
In the illustrated embodiment, the simulcast system 100 contains three simulcast channels. The comparator 122 and simulcast stations 130, 133, 136 (labeled as “CHL 1”) comprise a first simulcast channel, the comparator 124 and simulcast stations 131, 134, 137 (labeled as “CHL 2”) comprise a second simulcast channel and the comparator 126 and simulcast stations 132, 135, 138 (labeled as “CHL 3”) comprise a third simulcast channel. For each of the three simulcast channels, the associated comparator 122, 124, 126 sends signals to be transmitted to each of the simulcast stations of the respective simulcast channel. The simulcast stations of each simulcast channel then simultaneously transmit the signals on the same frequency. For example, the comparator 122 of the first simulcast channel sends signals to the simulcast stations 130, 133, 136 of the first simulcast channel. The simulcast stations 130, 133, 136 of the first simulcast channel then transmit the signals simultaneously on the same frequency. The simulcast stations of each of the simulcast channels may also simultaneously receive signals from a communication unit if they are within range. The received signals are sent from each of the simulcast stations that receive the signal to the associated comparator 122, 124, 126 of the simulcast channel. For example, if a communication unit communicating on the first simulcast channel is located within the overlap area 140, the simulcast station 130 at the remote site 107 and the simulcast station 133 at the remote site 109 would receive a signal sent by the communication unit. The two simulcast stations 130, 133 would then send the signal to the comparator 122 associated with the first simulcast channel.
Occasionally, one of the simulcast stations will become unavailable. This could happen, for example, because of a malfunction of the simulcast station or because of problems with the network connection between the simulcast station and the comparator associated with the same simulcast channel. Heretofore, when a simulcast station becomes unavailable, the simulcast site controller 120 would shut down the entire simulcast channel containing the unavailable simulcast station. This was done because the simulcast system 100 doesn't know where the communication units (not shown) are located and hence does not know if service can reliably be provided to the communication units.
As an example of the shutdown of the simulcast channels, assume that the two simulcast stations 130, 134 of the simulcast system 100 shown by an ‘X’ in FIG. 1 become unavailable. The unavailability of the simulcast station 130 of the remote site 107 would cause the shut down of the first simulcast channel and hence the other two simulcast stations 133, 136 of the first simulcast channel would stop functioning. The unavailability of the simulcast station 134 at the remote site 109 would cause the shut down of the second simulcast channel and hence the other two simulcast stations 131, 137 of the second simulcast channel would stop functioning. As a result, the unavailability of two of the simulcast stations 130, 134 would cause two thirds of the simulcast stations 130–138 in the simulcast system 100 to become idle. This is not an efficient use of the resources of the simulcast system 100.
Accordingly, there is a need for a means to increase the number of simulcast channels that remain operational when simulcast stations become unavailable. This invention is directed to addressing this need.